Outdoor air provides people with oxygen, which is essential to human life. The outdoor air we breathe can be polluted with chemicals from vehicles, electric power plants, incinerators, and other sources. Air pollution can also come from natural sources such as forest fires, industrial sources such as chemical plants and factories, and smaller sources such as dry cleaners and gas stations.

Air pollution can cause many harmful health effects, including asthma, heart disease, and cancer. Some air pollutants can be more harmful to human health than others. Some people, especially children and the elderly, are more vulnerable to the health effects of air pollution than others. Common air pollutants include ozone, which can cause asthma and respiratory problems; particulate matter, which can cause lung damage; sulfur dioxide, which can cause breathing problems; and volatile organic compounds, which can cause cancer.

Hazardous air pollutants are known or suspected to cause cancer, reproductive problems, birth defects, or other serious health effects. They are also called air toxics, or toxic air pollutants. The U.S. Environmental Protection Agency (EPA) is working to reduce the amounts of 188 toxic air pollutants, including asbestos, benzene, mercury, perchloroethylene, and toluene.

This description is based on the information found in the Web links listed with this topic.